With the advance of weight-, thickness- and length-reducing technology in the field of the electronic and electric industry and with the recent advancement of weight-reducing technology in the fields of the automobile, aircraft and space industres, there has been a strong demand for crystalline thermoplastic resins having heat resistance of about 300.degree. C. or higher and permitting easy melt processing in recent years.
As crystalline, heat-resistant, thermoplastic resins developed to date, there are, for example, poly(butylene terephthalate), polyacetal, poly(p-phenylene thioether), etc. These resins are however unable to meet the recent requirement level for heat resistance.
Polyether ether ketones (hereinafter abbreviated as "PEEKs") and polyether ketones (hereinafter abbreviated as "PEKs") have recently been developed as heat-resistant resins having a melting point of about 300.degree. C. or higher. These resins are crystalline thermoplastic resins. It has therefore been known that conventional melt processing techniques such as extrusion, injection molding and melt spinning can be applied to easily form them into various molded or formed products such as extruded products, injection-molded products, fibers and films.
These resins however use expensive fluorine-substituted aromatic compounds such as 4,4'-difluorobenzophenone as their raw materials. Limitations are thus said to exist to the reduction of their costs. It is also pointed out that these resins involve a problem in expanding their consumption.
Based on an assumption that poly(arylene thioether-ketones) (hereinafter abbreviated as "PTKs") could be promising candidates for heat-resistant thermoplastic resins like PEEKs and PEKs owing to their similarity in chemical structure, PTKs have been studied to some extent to date. There are some disclosure on PTKs, for example, in Japanese Patent Laid-Open No. 58435/1985 (hereinafter referred to as "Publication A"), German Offenlegungsschrift No. 34 05 523Al (hereinafter referred to as "Publication B"), Japanese Patent Laid-Open No. 104126/1985 (hereinafter referred to as "Publication C"), Japanese Patent Laid-Open No. 13347/1972 (hereinafter referred to as "Publication D"), Indian J. Chem., 21A, 501-502 (May, 1982) (hereinafter referred to as "Publication E"), and Japanese Patent Laid-Open No. 221229/1986 (hereinafter referred to as "Publication F").
Regarding the PTKs described in the above publications, neither molding nor forming has however succeeded to date in accordance with conventional melt processing techniques. Incidentally, the term "conventional melt processing techniques" as used herein means usual melt processing techniques for thermoplastic resins, such as extrusion, injection molding and melt spinning.
The unsuccessful molding or forming of PTKs by conventional melt processing techniques is believed to be attributed to the poor melt stability of the prior art PTKs, which tended to lose their crystallinity or to undergo crosslinking and/or carbonization, resulting in a rapid increase in melt viscosity, upon their melt processing.
It was attempted to produce some molded or formed products in Publications A and B. Since the PTKs had poor melt-stability, certain specified types of molded or formed products were only obtained by a special molding or forming process, where PTKs were used only as a sort of binder, being impregnated into a great deal of reinforcing fibers of main structural materials and molded or formed under pressure.
Since the conventional PTKs are all insufficient in melt stability as described above, it has been unable to obtain molded or formed products from them by applying conventional melt processing techniques.
The present inventors conducted an extensive investigation with a view toward developing a process for economically producing a PTK which has melt stability permitting the application of conventional melt processing techniques.
First of all, the present inventors chose economical dichlorobenzophenone and dibromobenzophenone as raw materials instead of employing expensive fluorine-substituted aromatic compounds. In addition, a polymerization process was designed in an attempt to conduct polymerization by increasing the water content in a polymerization system to an extremely high level compared to processes reported previously, adding a polymerization aid and suitably controlling the profile of the polymerization temperature. As a result, it was found that a high molecular-weight PTK would be obtained economically.
The high molecular-weight PTK obtained by the above new process was however still dissatisfactory in melt stability. As a next step, the present inventors made further improvements to the polymerization process. It was then revealed that PTKs, which were improved significantly in melt stability compared to the conventional PTKs and hence permitted the application of conventional melt processing techniques, can be obtained by conducting polymerization in a system free of any polymerization aid while paying attention to the selection of a charge ratio of monomers, the shortening of the polymerization time at high temperatures, the selection of a material for a polymerization reactor and optionally, by applying a stabilization treatment in a final stage of the polymerization. It was also found that molded and formed products such as extrusion products, injection-molded products, fibers and films would be obtained successfully and easily from such melt-stable PTKs by conventional melt processing techniques.
Even melt-stable PTKs permitting the application of these conventional melt processing techniques were however still unable to avoid certain degrees of thermal modification and deterioration when melt-processed from powdery polymers into pellets or molded or formed products, whereby they underwent melt viscosity increase and/or decrease of crystallinity and develop sticking of thermal decomposition products to resin residence areas of melt processing equipment. They hence involved a problem that difficulties were encountered in determining appropriate conditions for their melt processing.
It therefore arose, as a subject to be investigated, to make further improvements to the melt stability of the melt-stable PTKs upon their melt processing.
With a view toward preventing melt viscosity increase and/or decrease of crystallinity and sticking of thermal decomposition products to resin residence areas of melt processing equipment, the present inventors tried to improve physical properties of the above-described melt-stable PTKs by adding thereto various heat stabilizers known conventionally as heat stabilizers for poly(arylene thioethers) (PATEs), such as those to be described next.
Namely, it was attempted to make further improvements to the melt stability of the melt-stable PTKS upon melt processing by adding, as heat stabilizers, organic thiols (U.S. Pat. No. 3,386,950), organic hydroxylamines (U.S. Pat. No. 3,408,342), organophosphinic acids and organophosphites (U.S. Pat. No. 3,658,753), inorganic nitrites (U.S. Pat. No. 4,405,745), organotin compounds (U.S. Pat. No. 4,411,853), dithiocarbamates (U.S. Pat. No. 4,413,081), alkaline earth metal salts of fatty acids (U.S. Pat. No. 4,418,029), dithiophosphinic acid salts (U.S. Pat. No. 4,421,910), phenolamides and phenolesters (U.S. Pat. No. 4,434,122), aminotriazoles (U.S. Pat. No. 4,478,969), sorbic acid salts (U.S. Pat. No. 4,535,117 and U.S. Pat. No. 4,543,224), aromtic ketones, aromatic amines, aromatic amides and aromatic imides (U.S. Pat. No. 4,482,683), etc.
It was however unable to find anything among these compounds, which would show effects in improving the melt stability of the PTKs.